Many of the people who played the natives were indigenous South Americans who spoke little English. They were given free rein to say whatever lines they wanted. According to popular rumor, they are cursing up a storm in a few scenes.
The film was released four years after its uncredited source book "The Lost Cities of Paraguay" by Father C. J. McNaspy was published. McNaspy also acted as a historical consultant to the film, which was loosely based on McNaspy's work.
This is Ennio Morricone's favorite among his own movie scores. Morricone expressed disappointment that this film lost the 1987 Academy Award® for Best Music, Original Score to 'Round Midnight (1986), which included jazz standards. As of 2020, the Academy of Motion Picture Arts and Sciences requires nominated original scores to be at least 60% original, a minimum raised to 80% for sequels and franchise films.
A short scene after the closing credits features a final closeup of Cardinal Altamirano, which matches the opening closeup shot.
Daniel Berrigan: The elderly priest sometimes traveling in the background. Father Daniel Berrigan, S.J. is a well-known Jesuit priest and professor who, along with his brother, is known for his work in human rights and social justice. He has one line in the film, "No."